In general, a field coil assembly of an electromagnetic clutch is an electric device for transmitting driving force of a pulley rotating by an engine to a hub disc of a compressor driving shaft in such a manner that an electromagnetic field is formed by electromagnetic induction of a wound coil when power source is supplied, and the hub disc of the compressor driving shaft is attracted to the friction surface of the pulley by electromagnetic force and dynamically connected.
Furthermore, the field coil assembly serves to control operation of a cooling system of an air-conditioning apparatus by controlling driving power to the compressor according to whether or not to apply power source to the coil.
As shown in FIGS. 1 and 2, the field coil assembly of the electromagnetic clutch includes an electronic coil body 11 formed by winding an electronic coil in the form of a ring; a bobbin 13 for receiving the electronic coil body 11 and having a pair of lead wires 12 connected with a pair of extracted wires 11a drawn out from the electronic coil body 11, the lead wires 12 being exposed to the outside; a core ring 14 for receiving the bobbin 13 in which the electronic coil body 11 is contained, the core ring 14 being inserted and mounted into the pulley and fixed to the compressor 1; and a cover 15 for sealing the open surface of the core ring 14 to prevent separation of the electronic coil body 11 and the bobbin 13 from the core ring 14.
Each lead wire 12 has a connecting member (not shown) mounted at an end portion thereof to be connected with a connector 5 connected with the external power source, so that electromagnetic field is formed on the electronic coil body 11.
In the field coil assembly 10, when the power source applied to the electronic coil body 11 is cut off, the electronic coil body 11 generates surge voltage and counter electromotive force. When surge voltage and counter electromotive force are generated, it may have a bad influence, such as occurrence of short, on the peripheral electronic parts.
Therefore, a diode 21 is installed to the lead wires 12 of the electronic coil body 11 to prevent generation of surge voltage and counter electromotive force, and thereby, maintaining a normal condition of voltage.
Meanwhile, when the diode 21 is installed to the lead wires 12, a close circuit is formed by blocking surge voltage and counter electromotive force, and a weak attraction force acts due to residual electric current of the electronic coil body 11, so that a hub 4 and a disc 4a do not drop down rapidly from the friction surface 3a of the pulley 3 when power source is cut off.
That is, the hub 4 and the disc 4a drop down slowly from the friction surface 3a of the pulley 3 due to formation of the close circuit by the diode 21, and thereby, off-noise, which is friction noise among the hub 4, the disc 4a and the friction surface 3a of the pulley 3, is generated.
To prevent off-noise of the electromagnetic clutch, the diode 21 and a resistor 22 are connected to each other in series, and then, the resistor 22 is connected to the lead wires 12 in parallel, so that the resistor can absorb the residual electric current of the electronic coil body 11. Thereby, when power source is cut off, the weak attraction force acting to the electronic coil body 11 is removed, and off-noise of the electromagnetic clutch is prevented while the disc 4a of the hub 4 rapidly drops down from the friction surface 3a of the pulley 3.
As described above, to prevent generation of off-noise and surge voltage of the electromagnetic clutch, as shown in FIGS. 3 and 4, the diode 21 and the resistor 22 are connected to the lead wires 12 in parallel. At this time, the diode 21 and the resistor 22 are connected to each other in series by welding lead wires 23 extending from sides thereof with each other, lead wires 23a extending from the other sides of the diode 21 and the resistor 22 are connected to the lead wires 12 by splices 24.
To protect and insulate the field coil assembly from the outside, first, the splices 24 are covered with a covering material 25, second, the diode 21 and the resistor 22 are covered with other covering material 26, and finally, the covered parts are finished with a finish covering material 27.
Also, in a case in which only the diode 21 is connected to the lead wire 12, the above connecting and covering methods are applied in the same way.
Therefore, the conventional field coil assembly 10 has several problems in that a manufacturing cost is expensive due to lots of parts, and in that it is impossible to automatically manufacture the field coil assembly due to complicated manufacturing processes when only the diode 21 is mounted or the diode 21 and the resistor 22 are mounted.